Zwitterion-Ag Complexes That Simultaneously Enhance Biofouling Resistance and Silver Binding Capability of Thin Film Composite Membranes

Biofouling can be overcome with zwitterion grafts and antimicrobial, metallic nanoparticles. However, the mechanism underpinning this effective approach remains unclear. To elucidate the role of each component in this system while maximizing membrane antifouling and antimicrobial properties, here we performed a comparative study to investigate the impact of zwitterion type and their interactions with Ag of various states. Two different zwitterions (SO3--based and COO--based) were employed to modify polyamide (PA) thin film composite (TFC) membranes, and the metallized and mineralized membranes were developed via in situ formation of silver (Ag) nanoparticles and deposition of silver chloride (AgCl) particles on the zwitterion-modified TFC membranes. The presence of zwitterions was key to enhance Ag content, resulting in significantly improved antimicrobial and antifouling properties without compromising the nanofiltration separation performance. COO--based zwitterions were found more favorable toward Ag+ metallization and mineralization compared to SO3--based zwitterions. The underlying mechanisms underpinning this discovery were further revealed using density functional theory (DFT) to reveal Gibbs free energy of the binding between zwitterions and Ag+ ions. This fundamental knowledge is crucial for designing next-generation antibiofouling strategies.